I am confused how to understand the word "kāyanuttha" in the sentence: "kāyanuttha, bhikkhave, etarahi kathāya sannisinnā sannipatitā, kā ca pana vo antarākathā vippakatā’’ti" How would you see it formed? Thank you very much.

Mettā

"One should not consider the faults of others, nor their doing or not doing good or bad deeds. One should consider only whether one has done or not done good or bad deeds." -- The Buddha (Dhp.50)

kāya is the feminine of the interrogative ko, inflected in either the instrumental, dative, ablative, genitive or locative case. In the quoted passage I believe it’s instrumental: “With what [subject of discussion]?” Though the dative is also possible: “For the sake of what [subject of discussion]?” Or even the ablative of cause: “On account of what [subject of discussion]?”

nuttha (or var. nottha) is nu + attha

nu is a (usually untranslatable) interrogative, dubitative or emphatic particle.

attha is the verb atthi in either the present indicative second person plural:

atthi, santi
asi, attha
asmi/amhi, asma/amha

Or the imperative second person plural:

atthu, santu
āhi, attha
asmi/amhi, asma/amha

In the quoted passage it’s undoubtedly present indicative.

kāya nuttha etarahi kathāya sannisinnā

You are [attha] with what/for what/because of what [kāya nu] subject of discussion [kathāya] now [etarahi] seated together [sannisinnā]?

Bhikkhu Bodhi translates it:

“What discussion were you engaged in just now as you were sitting together here?”

Dhammanando wrote:You are [attha] with what/for what/because of what [kāya nu] subject of discussion [kathāya] now [etarahi] seated together [sannisinnā]?

Thank you, bhante, for your helpful analysis. A bit further I would like to ask, if you don't mind. Could ttha stand also as the shortened form of ettha (as Sujato has suggested)? Sannisinnā and sannipatitā agree with an implied tumhe as the nominative plural?

Mettā

"One should not consider the faults of others, nor their doing or not doing good or bad deeds. One should consider only whether one has done or not done good or bad deeds." -- The Buddha (Dhp.50)

A further consideration is that in Pali sandhi the vowel e tends to be strong and doesn't easily get zapped. If it were really nu + ettha, as Sujāto proposes, then the form that we should expect is n'v'ettha. As in the Kiṃsukopama Jātaka:

Sabbehi kiṃsuko diṭṭho, kiṃ nvettha vicikicchatha.

"The Judas tree has been seen by all; what herein do ye doubt?"

A. Bhikkhu wrote:Sannisinnā and sannipatitā agree with an implied tumhe as the nominative plural?